Drive circuit and physical quantity measuring device

ABSTRACT

In order for keeping the amplitude of the excitation current of a vibrator constant irrespective not only of the temperature variation but also of the manufacturing variation and the variation in frequency, a comparison control circuit for controlling the amplitude of the drive signal for exciting the vibrator includes a comparative voltage supply circuit for supplying the comparative voltage, and the comparative voltage supply circuit generates the comparative voltage with a constant current source and a second resistor made of a material the same as a material of a first resistor included in a current-voltage conversion circuit.

This is a Division of application Ser. No. 13/222,700 filed Aug. 31,2011, which claims priority to Japanese Patent Application No.2010-196933 filed Sep. 2, 2010, the disclosure of which is incorporatedherein by reference in its entirety.

BACKGROUND

1. Technical Field

The present invention relates to a drive circuit, a physical quantitymeasuring device, and so on.

2. Related Art

In general, a physical quantity measuring device using a vibratoroutputs a measurement signal proportional to the excitation current ofthe vibrator. In order for stable measurement, there is required a drivecircuit or the like for keeping the amplitude of the excitation currentconstant irrespective of the temperature variation in the measurementenvironment. In JP-A-2008-261844 (Document 1) there is disclosed aninvention of using an element identical to the resistor element of thedetection circuit in the drive circuit to thereby cancel out thetemperature variation between the excitation current and the detectioncurrent, thus stabilizing the output of the measurement signal.

However, as described in paragraphs 0060 through 0066 of Document 1, ifthe vibrational frequency of the quartz crystal vibrator is varied, theapplication thereof becomes difficult. According to Document 1, theproblem can be solved by making the resistance value also identical.However, since the circuit design constraints are severe, it is notrealistic. Further, the invention of Document 1 fails to go so far asconsidering the variation during the manufacturing process (hereinafterreferred to as a manufacturing variation). For example, the capacitanceof a capacitor, which is stable in the case of considering only thetemperature characteristics, can naturally include the manufacturingvariation. Therefore, in order for accurately detecting the minutevariation in the measurement signal, it is required to design the drivecircuit of the vibrator taking not only the temperature characteristicsbut also the manufacturing variation and the variation in frequency intoconsideration.

SUMMARY

According to some of the aspects of the invention, it is possible toprovide a drive circuit or the like for keeping the amplitude of anexcitation current of a vibrator constant irrespective not only of thetemperature variation but also of the manufacturing variation and thevariation in frequency.

(1) According to an aspect of the invention, there is provided a drivecircuit adapted to output a drive signal for exciting a vibratorincluding a current-voltage conversion circuit adapted to convert anexcitation current input into a voltage, a full-wave rectifying circuitadapted to perform full-wave rectification on an output voltage from thecurrent-voltage conversion circuit, a drive signal generation circuitadapted to generate the drive signal based on the output voltage fromthe current-voltage conversion circuit, and a comparison control circuitadapted to compare an output voltage from the full-wave rectificationcircuit with a comparative voltage to thereby control an amplitude ofthe drive signal, wherein the comparison control circuit includes acomparative voltage supply circuit adapted to supply the comparativevoltage, and the comparative voltage supply circuit generates thecomparative voltage with a constant current source and a second resistormade of a material the same as a material of a first resistor includedin the current-voltage conversion circuit.

According to this aspect of the invention, by controlling the amplitudeof the drive signal based on the comparative voltage, it is possible tokeep the amplitude of the excitation current of the vibrator constant.Firstly, since the first resistor included in the current-voltageconversion circuit and the second resistor included in the comparativevoltage supply circuit for supplying the comparative voltage are made ofthe same material, the rate of the variation in the resistance value dueto the temperature is the same. Therefore, the variation in theexcitation current in accordance with the temperature variation can beprevented.

Here, “the same material” denotes the material with which thecharacteristics such as the temperature characteristics or themanufacturing variation become the same between the first resistor andthe second resistor thus manufactured with the material. If, forexample, the temperature characteristics, the manufacturing variation,and so on become the same in the case in which the type (poly-siliconresistor, diffused resistor, well resistor, and so on) of the resistoris the same, the fact that the resistors are the same type can mean thatthe resistors are made of the same material. Further, it is alsopossible to determine whether or not the resistors are made of the samematerial taking not only the type of the resistor but also the localvariation into consideration.

Further, the drive circuit of the comparative example described laterhas the resistor corresponding to the second resistor as, for example, alow-pass filter of the compensation circuit. However, the drive circuitof the aspect of the invention does not require such a compensationcircuit, and therefore the design can be simplified, and the circuitscale is also reduced. Further, the capacitance of the capacitor of thelow-pass filter provided to the drive circuit of the comparative examplehas the variation due to the manufacturing variation although thetemperature variation is small. As a result, the variation in thecapacitance of the capacitor affects the excitation current, andtherefore, the difference in the excitation current occurs between theproducts including the drive circuit even if the temperature conditionis the same. The drive circuit according to the aspect of the inventioncan keep the amplitude of the excitation current of the vibratorconstant without causing such a problem.

Further, the gain of the drive circuit of the comparative example has afrequency dependency, and the excitation current tends to increase if,for example, the vibrational frequency is raised. However, in the drivecircuit according to the aspect of the invention, the gain does not havethe frequency dependency, and the amplitude of the excitation current ofthe vibrator can be kept constant.

According to the aspect of the invention, it is possible to provide adrive circuit for keeping the amplitude of the excitation current of thevibrator constant irrespective not only of the temperature variation butalso of the manufacturing variation and the variation in frequency. Itshould be noted that the term “constant” here does not mean that theamplitude is exactly equal to the amplitude of the excitation current asthe design target, but means that the amplitude is fit into thevariation range allowed under the use environment conditions defined bythe specification. The “variation range allowed” denotes the range, forexample, including the design target and smaller than the limit value.Further, the “limit value” denotes the amplitude of the excitationcurrent with which the breakage current of breaking, for example, thevibrator flows.

(2) In the drive circuit of the aspect of the invention, it is possiblethat the first resistor and the second resistor are disposed in the sameresistor area in an array fashion. According to this configuration, bydisposing the first resistor and the second resistor in the sameresistor area in the layout, it is possible to eliminate the localresistance variation to thereby keep the amplitude of the excitationcurrent constant eventually. On this occasion, since the first resistorand the second resistor are disposed in an array fashion, the way ofarrangement is the same, which makes it difficult to cause the variationin the resistance value due to the shape. Therefore, it is possible toalign the temperature characteristics between the first resistor and thesecond resistor.

(3) In the drive circuit of the aspect of the invention, it is possiblethat the first resistor and the second resistor are each composed of oneof a resistor cell and a combination of a plurality of resistor cells,the resistor cells each having the same resistance value.

According to this configuration, since the first resistor and the secondresistor are each composed of a combination of the resistor cells(resistor elements) in the layout, the variation in the resistance valuehardly occurs. Therefore, it is possible to align the temperaturecharacteristics between the first resistor and the second resistor.

(4) In the drive circuit of the aspect of the invention, it is possiblethat the resistance value of the first resistor and the resistance valueof the second resistor are determined based on a result of calculationof comparing an amplitude of the excitation current and a limit valuewith which a breakage current causing breakage of the vibrator flows.

According to this configuration, since the amplitude of the excitationcurrent as the design target can be approximated to the limit value asmuch as possible, it becomes possible to achieve improvement of the S/Nratio with the physical quantity measuring device using the drivecircuit, for example.

(5) In the drive circuit of the aspect of the invention, it is possiblethat the current-voltage conversion circuit and the comparison controlcircuit are connected to a common ground potential.

According to this configuration, the current-voltage conversion circuitincluding the first resistor and the comparison control circuitincluding the second resistor are connected to the common groundpotential to thereby prevent the difference in the temperaturecharacteristics between the both resistors from occurring, and thus, theamplitude of the excitation current can be kept constant. For example,the resistance values of the first resistor and the second resistor areoptimized under a certain condition (e.g., a “TYP” condition (typicalcondition)) in the design process. On this occasion, by connecting themto the common ground potential, it is possible to reduce the variationin the amplitude of the excitation current due to, for example, thetemperature variation as much as possible to thereby prevent theamplitude from reaching the limit value.

(6) According to another aspect of the invention, there is provided aphysical quantity measuring device including any of the drive circuitsaccording to the aspect of the invention described above.

According to the aspect of the invention, it is possible to provide aphysical quantity measuring device capable of performing stablemeasurement by including the drive circuit for keeping the amplitude ofthe excitation current of the vibrator constant irrespective not only ofthe temperature variation but also of the manufacturing variation andthe variation in frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a block diagram of a drive circuit according to a firstembodiment of the invention.

FIG. 2 is an example of a circuit diagram of the drive circuit accordingto the first embodiment.

FIG. 3A is a table showing conditions of a simulation. FIG. 3B is adiagram showing a variation in an excitation current in a drive circuitof a comparative example. FIG. 3C is a diagram showing a variation in anexcitation current in the drive circuit according to the firstembodiment.

FIG. 4A is a schematic diagram of a layout. FIG. 4B is a diagram showingan arrangement example of resistor cells in a resistor area. FIG. 4C isa diagram showing a connection example of the resistor cells shown inFIG. 4B.

FIG. 5 is a diagram showing a physical quantity measuring device of anapplication example.

FIG. 6 is a block diagram of a drive circuit of a comparative example.

FIG. 7 is an example of a circuit diagram of the drive circuit of thecomparative example.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Some embodiments of the invention will hereinafter be explained withreference to the accompanying drawings.

1. First Embodiment

A first embodiment of the invention will be explained with reference toFIGS. 1, 2, 3A through 3C, and 4A through 4C. Further, FIGS. 6 and 7 arealso referred to in the explanation of a comparative example.

1.1. Configuration of Drive Circuit of Present Embodiment

FIG. 1 is a block diagram of a drive circuit 10 according to the presentembodiment. The drive circuit 10 includes a current-voltage conversioncircuit 20, a full-wave rectifying circuit 30, a comparison controlcircuit 40, and a drive signal generation circuit 50.

The current-voltage conversion circuit 20 converts an excitation current200 from a vibrator 60 into a voltage to thereby output an outputvoltage 202. The amplitude of the output voltage 202 is proportional tothe amplitude of the excitation current 200.

The full-wave rectifying circuit 30 performs the full-wave rectificationon the output voltage 202 from the current-voltage conversion circuit 20to thereby obtain a roughly direct-current voltage, and thus outputs anoutput voltage 204.

The comparison control circuit 40 compares the output voltage 204 fromthe full-wave rectifying circuit 30 with a comparative voltage, and thenoutputs an output voltage 206 as a signal reflecting the comparisonresult to the drive signal generation circuit 50. Then, the drive signalgeneration circuit 50 controls the amplitude of a drive signal 208 basedon the output voltage 206.

The comparison control circuit 40 includes a comparative voltage supplycircuit 42 for supplying the comparative voltage. The comparativevoltage supply circuit 42 has a resistor (a second resistor) made of amaterial identical to the material of a resistor (a first resistor)included in the current-voltage conversion circuit 20, and generates thecomparative voltage with the second resistor. It is also possible togenerate the comparative voltage by, for example, a constant currentsource and the second resistor as in the present embodiment. Accordingto this configuration, it results that the same temperaturecharacteristics as that of the first resistor are reflected in thecomparative voltage. The drive signal generation circuit 50 generatesthe drive signal 208 based on the output voltage 202 from thecurrent-voltage conversion circuit 20.

It should be noted that the drive circuit 10 of the present embodimentcan include an input terminal (DG) 100 and an output terminal (DS) 102.On this occasion, the drive circuit 10 is used with the vibrator 60connected to the input terminal 100 and the output terminal 102. Thevibrator 60 can be, for example, a quartz crystal vibrator or a vibratorused for a physical quantity measuring device such as a gyro sensor.

1.2. Comparison with Drive Circuit of Comparative Example

Prior to showing an example of a circuit diagram of the drive circuitaccording to the present embodiment, a drive circuit of a comparativeexample will be explained with reference to FIGS. 6 and 7. Afterexplaining the drive circuit of the comparative example, the example ofthe circuit diagram of the drive circuit according to the presentembodiment is shown using FIG. 2, and is then compared.

1.2.1. Drive Circuit of Comparative Example

FIG. 6 is a block diagram of the drive circuit 500 of the comparativeexample. Differently from the drive circuit 10 (FIG. 1) of the presentembodiment, a compensation circuit 520 for compensating the variation inresistance is included, and a comparative voltage supply circuit 41 as aconstant voltage source having no association with the first resistor isused. It should be noted that the same constituents as those shown inFIG. 1 are provided with the same reference numerals, and theexplanation therefor will be omitted.

The compensation circuit 520 has a resistor corresponding to the secondresistor of the drive circuit 10 (FIG. 1) of the present embodiment. Inother words, the compensation circuit 520 has a resistor, which has thesame temperature characteristics as those of the first resistor includedin the current-voltage conversion circuit 20, for canceling out thetemperature variation of the first resistor.

The compensation circuit 520 receives the output voltage 202 from thecurrent-voltage conversion circuit 20. Further, the full-wave rectifyingcircuit 30 receives the output voltage 530 from the compensation circuit520 instead of the output voltage 202 from the current-voltageconversion circuit 20. FIG. 7 shows an example of a circuit diagram ofthe drive circuit 500 of the comparative example shown in FIG. 6. Itshould be noted that the same constituents as those shown in FIGS. 1 and6 are provided with the same reference numerals, and the explanationtherefor will be omitted.

The current-voltage conversion circuit 20 is composed of for example, acapacitor 22, a resistor 24, and an operational amplifier 26. Thecapacitance of the capacitor 22 is C_(iv), and the resistance value ofthe resistor 24 is R_(iv). The transfer function H_(iv) of thecurrent-voltage conversion circuit 20 is expressed by Formula (1). Itshould be noted that the resistor 24 corresponds to the first resistor.

$\begin{matrix}{{H_{iv}\left( {j\;\omega} \right)} = \frac{- R_{iv}}{1 + {{j\omega}\; C_{iv}R_{iv}}}} & (1)\end{matrix}$

Then, the gain component (the conversion gain) G_(iv) becomesapproximately R_(iv) if R_(iv)>>C_(iv) is fulfilled.G _(iv) =R _(iv)  (2)

The compensation circuit 520 includes a low-pass filter composed of aresistor 506 and a capacitor 508, and a noninverting amplifier circuitcomposed of resistors 502, 504, and an operational amplifier 510. Theresistance value of the resistor 506 is R_(c), the capacitance of thecapacitor 508 is C_(c), and the resistance values of the resistors 502,504 are R₂, R₁, respectively. The transfer function H_(co) of thecompensation circuit 520 is expressed by Formula (3).

$\begin{matrix}{{H_{co}\left( {j\;\omega} \right)} = {\frac{1}{1 + {j\;\omega\; C_{c}R_{c}}} \times \frac{R_{1} + R_{2}}{R_{2}}}} & (3)\end{matrix}$

Here, assuming the case in which the attenuation rate of the low-passfilter is 1/10, and the gain of the noninverting amplifier circuit is 10times, Formula (3) is simplified. Formula (4) is obtained by replacingthe gain in the noninverting amplifier circuit with a numerical value.

$\begin{matrix}{{H_{co}({j\omega})} = \frac{10}{1 + {j\;\omega\; C_{c}R_{c}}}} & (4)\end{matrix}$

The gain component G_(co) of the transfer function H_(co) expressed byFormula (4) is expressed by Formula (5).

$\begin{matrix}{G_{co} = \frac{10}{\sqrt{1 + \left( {\omega\; C_{c}R_{c}} \right)^{2}}}} & (5)\end{matrix}$

Following the assumption described above, if it is assumed that the gaincomponent G_(co) expressed by Formula (5) is 1, (ωC_(c)R_(c))²=99>>1 isobtained. Therefore, the gain component G_(co) is obtained as Formula(6).

$\begin{matrix}{G_{co} = \frac{10}{\omega\; C_{c}R_{c}}} & (6)\end{matrix}$

Here, advantages of the compensation circuit 520 will be explained. Thegain component G_(sub) with consideration for the current-voltageconversion circuit 20 and the compensation circuit 520 is obtained asFormula (7) from Formula (2) and Formula (6).

$\begin{matrix}{G_{sub} = {R_{iv} \times \frac{10}{\omega\; C_{c}R_{c}}}} & (7)\end{matrix}$

In Formula (7), R_(iv) and R_(c) exist in the numerator and thedenominator, respectively. Therefore, by using the material, which isthe same as that used in the resistor 24 (the resistance value ofR_(iv)) of the current-voltage conversion circuit 20, in the resistor506 (R_(c)) of the low-pass filter of the compensation circuit 520, theinfluence of the manufacturing variation in the resistance value and thetemperature variation is prevented. Therefore, the variation in theresistor 24 (the first resistor) can be absorbed by the compensationcircuit 520.

The full-wave rectifying circuit 30 performs the full-wave rectificationon the output voltage 530 from the compensation circuit 520 to therebyoutput the output voltage 204. Then, the output voltage 204 is smoothedby an integrator in the posterior stage, and therefore, a nearlydirect-current voltage can be obtained. The full-wave rectifying circuit30 inputs the output voltage 530 from the compensation circuit 520 intoa comparator 32 to thereby compare it with an analog ground potential.Further, switches 34, 35 are exclusively set ON using the output of thecomparator 32.

Here, the full-wave rectifying circuit 30 includes an invertingamplifier circuit composed of resistors 35, 37, and an operationalamplifier 38. Here, it is possible to assume that the resistance valuesof the resistors 36, 37 are equal to each other, and the gain is 1.Since the phase varies 180° due to the inverting amplifier circuit, bycombining it with the switches 34, 35, it results that only the positivepart of the sine wave is output as the output voltage 204.

On this occasion, the gain component G_(re) in the full-wave rectifyingcircuit 30 is expressed as Formula (8) from the average value of thesine wave.

$\begin{matrix}{G_{re} = \frac{2}{\pi}} & (8)\end{matrix}$

The comparison control circuit 40 includes an integrator composed of aresistor 46, a capacitor 47, and an operational amplifier 48. Thevoltage thus made to be direct-current obtained by the full-waverectifying circuit 30 is compared with the comparison voltage V_(ref0)from the comparative voltage supply circuit 41 as the constant voltagesource, and feedback is provided so that the voltage becomes equal toV_(ref0). Subsequently, the output voltage 206 from the operationalamplifier 48, which is the feedback signal, is input to the drive signalgeneration circuit 50. The output voltage 206 is used for controllingthe amplitude of the drive signal 208. The drive signal generationcircuit 50 includes a high-pass filter composed of a capacitor 52 and aresistor 53, a comparator 54, and an N-channel transistor 58 foroutputting the drive signal 208. The high-pass filter for receiving theoutput voltage 202 from the current-voltage conversion circuit 20adjusts the phase. The output of the high-pass filter is input to thecomparator 54. Subsequently, the output is compared with the analogground potential and is then output as a rectangular wave. When theN-channel transistor 58 is switched. ON due to the rectangular wave, thedrive signal 208 becomes in a low level. In contrast, if the N-channeltransistor 58 is switched OFF, the drive signal 208 becomes in a highlevel. Here, since the high level of the drive signal 208 reflects theoutput voltage 206 of the comparison control circuit 40 connected to thedrain of the N-channel transistor 58 via the resistor 56, the amplitudeof the drive signal 208 is controlled.

1.2.2. Problems of Drive Circuit of Comparative Example

Denoting the amplitude of the excitation current 200 with I_(dr), in thedrive circuit 500 of the comparative example, there is a relationship ofFormula (9) between the amplitude I_(dr) and the comparative voltageV_(ref0) according to Formula (7) and Formula (8).

$\begin{matrix}{V_{{ref}\; 0} = {I_{dr} \times R_{iv} \times \frac{10}{\omega\; C_{c}R_{c}} \times \frac{2}{\pi}}} & (9)\end{matrix}$

Formula (10) is obtained by transforming Formula (9) into an expressionfor the amplitude I_(dr) of the excitation current 200.

$\begin{matrix}{I_{dr} = {\left( {\frac{\pi}{20} \times V_{{ref}\; 0}} \right) \times \omega\; C_{c} \times \frac{R_{c}}{R_{iv}}}} & (10)\end{matrix}$

As described above, the temperature variations and so on of theresistances R_(c) and R_(iv) cancel each other out. Further, thecomparative voltage V_(ref0) is a constant voltage, and the value in thebracket of the right-hand side of Formula (10) is a constant. Therefore,since the temperature variation of the capacitance C_(c) is sufficientlysmall as long as it is used at the same frequency (ω) even if thetemperature of the use environment of the drive circuit 500 of thecomparative example varies, the amplitude I_(dr) of the excitationcurrent 200 is kept constant.

However, the capacitance C_(c) also includes a variation due to themanufacturing variation. Further, it can usually occur that the circuitis used at a plurality of frequencies. Therefore, taking not only thetemperature variation but also the manufacturing variation and thevariation in frequency into consideration, it is difficult for the drivecircuit 500 of the comparative example to obtain the constant excitationcurrent 200.

1.2.3. Solution of Problems by Drive Circuit of Present Embodiment

FIG. 2 shows an example of a circuit diagram of the drive circuit 10 ofthe present embodiment. It should be noted that the same constituents asthose shown in FIGS. 1, 6, and 7 are provided with the same referencenumerals, and the explanation therefor will be omitted.

In the drive circuit 10 of the present embodiment, the compensationcircuit 520 is eliminated from the drive circuit 500 (FIGS. 6 and 7) ofthe comparative example, and the comparative voltage supply circuit 42is provided with a resistor 44 (the second resistor), which functions soas to absorb the variation in the resistor 24 (the first resistor). Thecomparative voltage supply circuit 42 supplies the comparative voltagegenerated by the constant current source 43 and the resistor 44.Denoting the current value of the constant current source with I_(ref),and the resistance value of the resistor 44 with R_(ref), thecomparative voltage V_(ref) is expressed as Formula (11).V _(ref) =I _(ref) ×R _(ref)  (11)

The comparative voltage supply circuit 41 of the drive circuit 500 ofthe comparative example supplies the comparative voltage as a constantvoltage. The comparative voltage supply circuit 42 of the drive circuit10 of the present embodiment is different therefrom in the point thatthe comparative voltage also varies with, for example, the temperaturevariation of the resistance value R_(ref) of the resistor 44. Here, whencorrecting Formula (9) and taking Formula (11) into consideration, thereexists the relationship of Formula (12) between the amplitude I_(dr) ofthe excitation current 200 and the comparative voltage V_(ref) withrespect to the drive circuit 10 according to the present embodiment.

$\begin{matrix}{{I_{ref} \times R_{ref}} = {I_{dr} \times R_{iv} \times \frac{2}{\pi}}} & (12)\end{matrix}$

Formula (13) is obtained by transforming Formula (12) into an expressionfor the amplitude I_(dr) of the excitation current 200.

$\begin{matrix}{I_{dr} = {\left( {\frac{\pi}{2} \times I_{ref}} \right) \times \frac{R_{ref}}{R_{iv}}}} & (13)\end{matrix}$

As expressed in Formula (10), in the drive circuit 500 of thecomparative example, the amplitude I_(dr) is affected by the vibrationalfrequency (ω) and the capacitance C_(c) of the low-pass filter. However,as expressed in Formula (13), the drive circuit 10 of the presentembodiment does not include the vibrational frequency ω and thecapacitance C. Therefore, the constant amplitude I_(dr) can be obtainedas long as the temperature variations and so on of the resistancesR_(ref) and R_(iv) cancel each other out. It should be noted that it ispreferable to connect the current-voltage conversion circuit 20 and thecomparison control circuit 40 to a common ground potential in order formaking the temperature characteristics of the resistances R_(ref) andR_(iv) identical to each other.

As described above, it is possible for the drive circuit 10 of thepresent embodiment to keep the amplitude of the excitation current of avibrator constant irrespective not only of the temperature variation butalso of the manufacturing variation and the variation in frequency.

1.2.4. Advantages of Drive Circuit of Present Embodiment

The advantages of the drive circuit according to the present embodimentwill be explained with reference to FIGS. 3A through 3C. FIG. 3A is atable showing conditions of a simulation. FIGS. 3B and 3C are diagramsshowing the variation in the excitation current in the drive circuit ofthe comparative example, and that in the drive circuit according to thefirst embodiment, respectively.

In the case, for example, in which the drive circuit shown in FIG. 2 orFIG. 7 is applied to a physical quantity measuring device, it isrequired to keep the amplitude of the excitation current constant inorder for stably measuring the measurement signal proportional to theexcitation current. Further, it is required to increase the amplitude ofthe excitation current to thereby improve the S/N ratio in order formeasuring the measurement signal with accuracy. On the other hand, ifthe amplitude of the excitation current is made excessively large, itresults that the vibrator is operated with excessive electrical power,which causes breakage such as a broken vibrator. Here, the current inthe case of causing the breakage of the vibrator is defined as abreaking current, and the amplitude of the excitation current with whichthe breaking current is caused is defined as a limit value.

In such a case, it is required that the amplitude I_(dr) of theexcitation current is smaller than the limit value even when taking thetemperature variation, the manufacturing variation, and the variation infrequency into consideration, and at the same time, is as large aspossible in view of the improvement of the S/N ratio. In the design ofthe drive circuit, for example, the resistance values of the firstresistor and the second resistor are tentatively determined in the TYPcondition based on Formula (13) and past data, and then adjusted basedon the estimation on how much margin exists to the limit value by thesimulation. FIGS. 3B and 3A show an example of the result of thesimulation on that occasion. Here, the TYP condition denotes a typicalcondition, and is a condition in which the temperature is ordinarytemperature of, for example, 25° C., and a process parameter takes astandard value.

In this example, the simulation conditions 1 through 4 are determined asshown in FIG. 3A, and as the number of the condition increases, thenumber of objects to be considered increases, and therefore, thecondition becomes severer. For example, in the condition 1, only thevariation in the resistance value of each of the first resistor and thesecond resistor (the resistor included in the low-pass filter in thedrive circuit of the comparative example) in accordance with thevariation (e.g., −45° C. through 80° C.) of the use temperature isconsidered. On and after the condition 2, the variation in thecapacitance due to the manufacturing variation is further considered,and on and after the condition 3, the vibrational frequency is sethigher. Here, “f₀” denotes the lowest frequency determined by thespecification, and “f₂” denotes the highest frequency, for example.Further, it is assumed that “f₁” is a frequency interveningtherebetween. FIG. 3B shows the simulation result in the case in whichthe resistances R_(c) and R_(iv) are determined so that the amplitudeI_(dr) of the excitation current becomes equal to a target value T inthe TYP condition in the drive circuit 500 of the comparative exampleshown in FIG. 7. In the condition 1, the range of the variation in theamplitude I_(dr) indicated by the arrow is small, and the maximum valuethereof is also smaller than the limit value F. Since the temperaturevariations of the respective resistances R_(c), R_(iv) cancel each otherout, the expected result can be obtained.

However, as expressed by Formula (10), the amplitude I_(dr) in the drivecircuit 500 of the comparative example is affected by the variation inthe frequency (m) and the capacitance C_(c). Therefore, in the condition2, the range of the variation in the amplitude I_(dr) is larger.Further, in the conditions 3 and 4, the level and the range of thevariation in the amplitude I_(dr) increase with the variation in thefrequency, and in some cases, the amplitude I_(dr) exceeds the limitvalue F. Therefore, judging from the simulation result, redesigning forsetting the amplitude I_(dr) lower than the target value T in the TYPcondition becomes necessary. This means the fact that the S/N ratio isreduced and the accuracy of measurement can be degraded if the drivecircuit 500 of the comparative example is applied to the physicalquantity measuring device.

FIG. 3C shows the simulation result in the case in which the resistancesR_(ref) and R_(iv) are determined so that the amplitude I_(dr) of theexcitation current becomes equal to a target value T in the TYPcondition in the drive circuit 10 of the present embodiment shown inFIG. 2. Since the temperature variations of the respective resistancesR_(ref), R_(iv) cancel each other out, and therefore, the amplitude isnot affected by the variation in the frequency (ω) and the capacitanceC_(c) differently from the case of FIG. 3B, the expected result can beobtained in all of the conditions 1 through 4.

Moreover, there is sufficient margin between the maximum value of thevariation and the limit value F, and there is a room for furtherincreasing the amplitude I_(dr). In other words, it is possible to shiftthe target value from “T” to a new target value T_(n) (>T). On thisoccasion, the resistance values of the first and second resistors arenewly determined by substituting T_(n) into the left-hand side ofFormula (13). This means the fact that the S/N ratio can be raised andthe accuracy of measurement can be improved if the drive circuit 10according to the present embodiment is applied to the physical quantitymeasuring device.

1.3. Homogenization of Material of Resistors

In the drive circuit according to the present embodiment, it is requiredfor the first and second resistors to have the same temperaturecharacteristics and to show the same manufacturing variation. Therefore,the layout process for homogenizing the material of the first and secondresistors will be explained with reference to FIGS. 4A through 4C. Itshould be noted that in the present embodiment, the homogenization ofthe material means not only to uniform the type of the resistor but alsoto make the characteristics identical including the local variation(e.g., unevenness in the sheet resistance due to the variation in theamount of implantation of ion in the ion-implantation process) due tothe difference in location in the layout.

FIG. 4A is a schematic diagram of a layout 300 of the drive circuit 10(see FIG. 2) according to the present embodiment. A cell 302 is a pad ora node of an input terminal DG, and a cell 304 is a pad or a node of anoutput terminal DS. Resistor areas 310, 312, and 320 are each an areawhere resistors are disposed, and resistor cells are mainly arranged inan array fashion in each of the resistor areas. Here, “in an arrayfashion” denotes that the same resistor cells are arranged at certainintervals as shown in FIG. 4B. It should be noted that dummy cells canbe disposed therebetween, and it is not required that the distancesbetween the adjacent resistor cells are the same.

As shown in the left part of FIG. 4A, the first resistor (the resistor24 with the resistance value of R_(iv) shown in FIG. 2) included in thecurrent-voltage conversion circuit 20 for receiving the signal from theinput terminal DG tends to be disposed in the resistor area 310 in thevicinity of the cell 302 of the input terminal DG. Further, the secondresistor (the resistor 44 with the resistance value of R_(ref) shown inFIG. 2) for generating the comparative voltage related to the drivesignal output from the output terminal DS tends to be disposed in theresistor area 312 in the vicinity of the cell 304 of the output terminalDS. In this case, there is a possibility of causing the local variationin the first and second resistors, and in particular, the possibilityrises as the area of the drive circuit increases. Therefore, the localvariation is prevented from occurring by disposing the first and secondresistors in the same resistor area such as the resistor area 320 asshown in the right part of FIG. 4A.

FIG. 4B is a diagram showing an arrangement example of resistor cells322A through 322J in the resistor area 320. In this example, each of theresistor cells 322A through 322J is a resistor element of the same typehaving the same resistance value. It should be noted that FIG. 43represents a top view in a certain layer, and shows only a part of theresistor cell. The resistor cells 322A through 322J are connected tometal or the like in another layer via through holes or the like, andrealize various resistance values using series connection or parallelconnection. If the resistance values R_(iv) and R_(ref) are formed usingthe combinations of the resistor cells 322A through 322J, there is onlya little chance to cause the local variation since these resistor cellsare disposed in the narrow area in the same fashion.

FIG. 4C is a diagram showing a connection example of the resistor cellsshown in FIG. 43. It is assumed, for example, that R_(ref)=47.5 kΩ andR_(iv)=15 kΩ are required from Formula (13) in the case in which each ofthe resistor cells has the resistance value of 15 kΩ. In this case, theresistance value R_(ref) can be formed by connecting the resistor cells322A through 322C in series to each other (45 kΩ), and then furtherconnecting them to the resistor cells 322E through 322J (2.5 kΩ)connected in parallel to each other. Further, the resistor cell 322Dintervening therebetween can be used as the resistance value R_(iv). Asdescribed above, by forming both of the resistance values R_(ref) andR_(iv) with the combinations of the same resistor cells, the possibilityof causing the variation in the characteristics can be reduced. Further,by disposing the resistor cells constituting the resistance value R_(iv)and the resistor cells constituting the resistance value R_(ref) in adispersed manner, the possibility of causing the local variation canfurther be reduced. In particular, in the case in which the resistancevalue R_(iv) is composed of a plurality of resistor cells, such resistorcells and the resistor cells constituting the resistance value R_(ref)can be disposed alternately.

2. Application Example

An application example of the drive circuit according to the inventionwill be explained with reference to FIG. 5. The same constituents asthose shown in FIGS. 1, 2, 3A through 3C, and 4A through 4C are providedwith the same reference numerals, and the explanation therefor will beomitted.

2.1. Physical Quantity Measuring Device

FIG. 5 is a diagram showing an example of the physical quantitymeasuring device 1. The physical quantity measuring device 1 includesthe drive circuit 10, a detection circuit 90, the vibrator 60, and avibrator 70. The detection circuit 90 receives measurement signals 290,292 as, for example, differential signals from a detection sectionprovided to the vibrator 70. The measurement signals are proportional tothe excitation current of the vibrator 60. Then, the detection circuit90 receives necessary information from the drive circuit 10 as aninternal signal 294, then performs a predetermined calculation, and thenoutputs the detection signal 296 corresponding to the physical quantityto be measured.

The physical quantity measuring device 1 is, for example, a gyro sensor,and can output the angular velocity thus detected as the detectionsignal 296. Further, the physical quantity measuring device 1 is, forexample, an acceleration sensor, and can output the acceleration thusdetected as the detection signal 296. On this occasion, it is alsopossible that arithmetic processing is performed in the detectioncircuit 90, and the velocity information is output as the detectionsignal 296, or the distance information is output as the detectionsignal 296.

Due to the advantages of the drive circuit 10 according to the presentembodiment described with reference to FIGS. 3A through 3C, the physicalquantity measuring device 1 is capable of stably performing themeasurement irrespective of the temperature variation, the manufacturingvariation, and the variation in frequency, and thus performing themeasurement with accuracy because of the high S/N ratio.

Besides these exemplifications, the invention includes configurations(e.g., configurations having the same function, the same way, and thesame result, or configurations having the same object and the sameadvantages) substantially the same as any one of the configurationsdescribed in the embodiment section. Further, the invention includesconfigurations obtained by replacing a non-essential part of theconfigurations described in the embodiment section. Further, theinvention includes configurations exerting the same advantages orconfigurations capable of achieving the same object as theconfigurations described in the embodiment section. Further, theinvention includes configurations obtained by adding technologies knownto the public to the configurations described in the embodiment section.

What is claimed is:
 1. A drive circuit adapted to output a drive signalfor exciting a vibrator, comprising: a current-voltage conversioncircuit adapted to convert an excitation current input into a voltage; arectifying circuit adapted to perform rectification on an output voltagefrom the current-voltage conversion circuit; a drive signal generationcircuit adapted to generate the drive signal based on the output voltagefrom the current-voltage conversion circuit; and a comparison controlcircuit adapted to compare an output voltage from the rectifying circuitwith a comparative voltage to thereby control an amplitude of the drivesignal, wherein the comparison control circuit includes a comparativevoltage supply circuit adapted to supply the comparative voltage, andthe comparative voltage supply circuit generates the comparative voltagewith a constant current source and a second resistor made of materialhaving a temperature characteristic the same as a temperaturecharacteristic of a first resistor included in the current-voltageconversion circuit.
 2. The drive circuit according to claim 1, whereinThe first resistor and the second resistor are disposed in the sameresistor area in an array fashion.
 3. The drive circuit according toclaim 1, wherein The first resistor and the second resistor are eachcomposed of one of a resistor cell and a combination of plurality orresistor cells, the resistor cells each having the same resistancevalue.
 4. The drive circuit according to claim 1, wherein The resistancevalue of the first resistor and the resistance value of the secondresistor are determined based on a result of calculation of comparing anamplitude of the excitation current and a limit value with which abreakage current causing breakage of the vibrator flows.
 5. The drivecircuit according to claim 1, wherein The current-voltage conversioncircuit and the comparison control circuit are connected to a commonground potential.
 6. A physical quantity measuring device comprising thedrive circuit according to claim
 1. 7. A physical quantity measuringdevice comprising the drive circuit according to claim
 2. 8. A physicalquantity measuring device comprising the drive circuit according toclaim
 3. 9. A physical quantity measuring device comprising the drivecircuit according to claim
 4. 10. A physical quantity measuring devicecomprising the drive circuit according to claim 5.